The Centers for Disease Control and Prevention said Friday that E. coli-contaminated ground beef was the suspected culprit in an outbreak that infected at least 109 people in six states.

Investigators are a step closer to tracking down the source of contamination that has perplexed them since health officials in Kentucky and Georgia notified them on March 28. The CDC said 17 people have been hospitalized, though no deaths have been reported.

Health officials said they were still working to trace the source of tainted meat from stores and restaurants where people ate. No common source has been publicly identified.

Reported illnesses come from states that share at least one border with another in this group: Indiana, Ohio, Kentucky, Virginia, Tennessee and Georgia.

The CDC is not recommending that people stop eating ground beef or that retailers stop selling it. But health officials have reminded people to handle raw ground beef safely and to cook it thoroughly.

Symptoms of E. coli infection often include severe stomach cramps, diarrhea and vomiting, typically occurring three days after consumption of the bacteria.

This particular strain, E. coli O103, has been a relatively infrequent source of outbreaks. In 2010, an outbreak in Minnesota sickened 29 — one contamination among nearly 20 in the past two decades.

State health officials interviewed dozens of people affected in the recent outbreak to determine its source. The widening dragnet indicates that health officials struggled at first to determine its source. “Given the size and the number of states that are involved, what you’re seeing is very unusual,” Bill Marler, a food safety lawyer from Seattle, previously told The Washington Post.

Marler said the CDC estimates that for every person reported sick, there are five to 10 ill people who are ill but not accounted for.

Major outbreaks of food-borne infections are detected three times as often than they were 20 years ago, which the CDC attributes to better efforts to find and investigate them. From 2010 to 2014, the most recent five-year period in the CDC’s report, these multistate outbreaks were bigger and deadlier than in years past, causing more than half of all deaths related to contaminated-food outbreaks, according to a 2015 CDC report. The germs most frequently implicated are familiar to most Americans: salmonella, E. coli and listeria.

Tainted ground beef has been among the culprits. Authorities recalled a total of about 12 million pounds of raw beef products last year after more than 400 people in 30 states fell ill from salmonella infection. More than 100 people were hospitalized.

Other foods, including vegetables, fresh fruits and chicken, have also helped spread infections nationwide. Some had never before been linked to outbreaks. Among those were listeria-tainted caramel apples that led to an outbreak in which seven people died and 34 were hospitalized in late 2014.

Condiments, garnish, toppings and spices can all contain traces of E. coli. But it is unlikely the patients in this outbreak were keeping track of all the additives in their recent diets, Marler told The Post.

Alex HortonAlex Horton is a general assignment reporter for The Washington Post. He previously covered the military and national security for Stars and Stripes, and served in Iraq as an Army infantryman. Follow

Michael Brice-SaddlerMichael Brice-Saddler is a general assignment reporter. He joined The Washington Post in June 2018 as an intern after graduating from the University of Maryland at College Park. Follow

Lena H. SunLena H. Sun is a national reporter for The Washington Post covering health with a special focus on public health and infectious disease. A longtime reporter at The Post, she has covered the Metro transit system, immigration, education and was a Beijing bureau chief. Follow

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